CZ298912B6 - Scanning electron microscope - Google Patents

Abstract

The scanning electron microscope with a secondary electron detector has a magnetic object glass near the two-field lenses located on the specimen being scanned. The detector is between the two two-field lenses.

Description

Scanning electron microscope

Technical field

The present invention relates to the field of high resolution scanning electron microscopy with a large field of view.

BACKGROUND OF THE INVENTION

One method of increasing the resolution of the scanning electron microscope is to reduce the working distance between the objective and the sample. In a conventional microscope design, this is limited by the secondary electron detector being adjacent to the objective and sample, generally between them. If the working distance is too small, the detector no longer has a sufficient signal.

Known solutions overcome this drawback by placing a secondary electron detector either directly into or above the objective. This allows the working distance to be reduced without losing the signal. For example, U.S. Pat. No. 4,831,266 or U.S. Pat. No. 5,387,793 disclose such solutions with a simple conventional lens or a single immersion lens. However, both of these arrangements do not allow to operate in different display modes, see below. Moreover, the latter has the disadvantage of greatly reduced field of view, since the scanning coils are located above the lens and the detection device, so that the scanning angle is small.

In contrast, in conventional microscopes using a two-lens objective, multiple imaging modes can be used, such as high-resolution first-lens imaging, high-depth second-lens imaging and large field of view, or a combination of both. For example, conventional microscopes have been manufactured by Tescan for many years. As mentioned, their disadvantage is the loss of signal over short working distances.

US-A-6,894,277, which uses dual lenses in the lens and a secondary electron detector positioned above the lens, attempts to combine both approaches so that it can operate at small working distances while alternating between the first and second lenses or combining them. However, this arrangement has the disadvantage of a relatively small field of view, since the scanning coils are located above the two objective lenses, so that the scanning angle is small. Furthermore, it has the disadvantage that for the operation of the device it is necessary to have a sample at a negative potential, which limits the use of the microscope.

U.S. Patent No. 5,578,822 also uses dual lenses in the lens, but the first is single-pole and the second is double-pole. However, a single-pole lens presents problems in imaging magnetic samples because its magnetic field reaches to the sample and is distorted by it. Furthermore, due to the shape of the magnetic field, this single-pole lens allows the detection of secondary electrons from only a small field of view, in which a width of 0.1 mm is patented, which limits the possible combined use of both lens lenses for large field imaging.

SUMMARY OF THE INVENTION

The above drawbacks are overcome by a scanning electron microscope comprising a primary electron source, means for accelerating primary electrons to the desired kinetic energy, a magnetic objective having an optical axis, a scanning electron beam scanning system for the sample and a secondary electron detector according to the present invention. The essence of the new solution is that the magnetic lens consists of the first two-pole lens placed closer

To the measured sample and lying away from the sample at a first working distance and from a second, two-pole lens lying on a common optical axis with a second working distance from the sample. A secondary electron detector is located between the first two-pole lens and the second two-pole lens.

In one possible embodiment, adjacent polar electron detector openings are provided in the pole pieces of the first two-pole lens. A first hole is formed in the outer pole piece and a second hole is formed in the inner pole piece.

In a preferred embodiment, further apertures are formed in the pole pieces of the first two-pole lens, such that at least one third aperture is formed in the outer pole piece and at least one fourth aperture is formed in the inner pole piece. The third apertures are of the same shape and size as the first aperture and the number thereof is including the first aperture n. The fourth apertures are the same shape and size as the second aperture and their number including the second aperture is m, n and m being even numbers. All openings in the outer pole piece, as well as all openings in the inner pole piece, are symmetrically spaced around the optical axis of the objective.

In another possible embodiment, the scanning system is located between the first two-pole lens and the second two-pole magnetic objective lens, and may preferably lie above the laden electron secant detector 20.

In another embodiment, an extraction electrode is provided in the magnetic objective and is connected to a source of positive, preferably variable, voltage. The voltage across the pole pieces of the magnetic lens is zero. Above the extraction electrode, at least one retarding electrode 25 may be disposed, which is connected to a source of low negative voltage, or may be aperture-shaped and grounded. In the latter case, a grounded shielding sleeve may be placed above the retarding electrode.

In another possible embodiment, a target having a hole for the passage of the primary electrons is located above the plane of the second orifice.

It is also possible to place between the extraction electrode and the plane of the second aperture a device for generating a crossed magnetic and electrostatic field, preferably adjustable, perpendicular to the optical axis of the objective.

In all these modifications, it is also possible to place a deflector in front of the secondary electron detector (5), for example a grid or a deflection electrode, which deflector is connected to a positive voltage source.

Further, it is possible to insert a backscattered electron detector and / or a cathodoluminescence detector and / or an X-ray detector into at least one of the pairs of adjacent third aperture and fourth aperture. These detectors can be pulled into the optical axis region or slid behind the wall of the inner pole piece.

In yet another embodiment, the first bipolar lens and the second bipolar lens have connected magnetic circuits.

In all embodiments, the sample may be connected to a negative voltage source.

This arrangement has the advantage that it allows working at short working distances in different display modes and the sample does not need to be negatively charged. For example, the first lens closer to the sample can be imaged at very low working distances for high resolution; in addition, the second lens can be displayed over longer working distances for a larger field of view and a greater depth of field; further, it is possible to combine images of both, for example by focusing the second lens at infinity (parallel beam) and first focusing on the sample; etc.

-2GB 298912 B6

Furthermore, this arrangement has the advantage that the rasters are positioned between the two lenses, i.e. closer to the sample and thus the field of view is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

Giant. 1 shows a possible embodiment of a microscope according to this solution.

Giant. 2 shows another possible embodiment of the microscope.

FIG. 3 shows yet another possible embodiment of the microscope.

Giant. 4 shows yet another possible embodiment of the microscope.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is a schematic cross-section of a scanning electron microscope where source I emits primary electrons accelerated to kinetic energy typically 0.5-50 keV, which pass through a condenser 2 and are focussed on a sample through a magnetic objective 3. or the thermo-emitting nozzle and condenser 2 may have any convenient shape. Another component of the scanning electron microscope is the scanning system 6, which ensures scanning of the primary electron beam over the sample surface. For example, as shown in FIG. 1, it may consist of two levels of scanning coils. In the present example, a stigmator 9 is used between the condenser 2 and the magnetic objective 3 for correcting aberrations. These stigmators 9 may also be in another suitable position, for example within the magnetic objective 3.

The magnetic objective 3 consists of a first two-pole lens 3a whose center has a first working distance WD1 from sample 4 and a second two-pole lens 3b whose center has a second working distance WD2 from sample 4. Both lenses are of conventional shape, so as mentioned above they are bipolar. Two lenses allow you to work in different display modes. For example, the first two-pole lens 3a can be imaged at very low operating distances for high resolution; further, the second two-pole lens 3b can be imaged at longer working distances for a larger field of view and a greater depth of field; further, it is possible to combine the imaging of both, for example, with a second double-pole lens 3b to focus on infinity (parallel beam) and to first focus on the sample; etc.

For better detection of secondary electrons, there is an extraction electrode 7 in the magnetic objective 3, which is connected to a source of positive, preferably variable, voltage. It is located above the outer pole of the first two-pole lens 3a, it may be either completely above the lens 3a, or even overlap and partially extend into the gap between its pole pieces. The secondary electrons emitted from Sample 4 upon impact of the primary electron beam are drawn into the magnetic object 40 through the extraction electrode 7. Both the magnetic lens 3 and Sample 4 are grounded and the extraction electrode 7 has a positive voltage of hundreds of volts, for example 300V. negative voltage, but the secondary electron detection method described below will not change. When imaging by the first two-pole lens 3a is activated, the secondary electrons upon focusing the gap between the pole pieces of the first two-pole lens 3a are focused by its magnetic field and advance in a spiral motion up along the optical axis. The secondary electron detector 5 is positioned laterally and inserted through a first aperture 3c and a second aperture 3d formed in the pole pieces of the first bipolar lens 3a. For example, the detector 5 may consist of a scintillation crystal, a light guide and a photomultiplier. A positive voltage of about 10 kV is applied to the end of the scintillation crystal detector 5, attracting secondary electrons to the detector 5 as shown schematically in Fig. 1. The signal from the detector 5 is processed and displayed on the terminal 5a, such as a CRT screen (Cathode Ray Tube).

-3 CZ 298912 B6

The secondary electrons emitted by Sample 4 have kinetic energy in units or at most tens of eV. As they pass through the extraction electrode 7, they are accelerated by its voltage and then again, due to the earthed walls of the magnetic objective 3, slowed down to their original kinetic energy. Opposite the second aperture 3d with the detector 5, the secondary electrons sense its voltage and are accelerated towards the detector 5. The field strength of the detector 5 can be controlled by the distance of the end of the detector 5 from the optical axis or the magnitude of its voltage. In case the geometric dimensions of the magnetic objective 3 do not allow sufficient distance between the extraction electrode 7 and the detector 5, ie that on the optical axis opposite the detector 5 the field from the extraction electrode 7 would be too strong, it is still possible between the extraction electrode 7 and the plane insert a retario electrode 8 to shield the extraction electrode 7, for example by applying a negative low voltage or by narrowing the inner diameter, but not to limit the primary beam scanning over the sample 4; in this case, the retarding electrode 8 may also be grounded and will have the shape of an orifice.

The first and second apertures 3c and 3d in the pole pieces, which serve for the detector 5 of the secondary electrons 15, cause the rotational asymmetry of the magnetic flux through the magnetic objective 3 and hence the optical defects in the image. For design reasons, the opening 3c may be of a different size than the opening 3d and both may be at a somewhat different height. For compensating the optical defects to a negligible part, it is possible to drill additional holes in the pole pieces of the first two-pole lens 3a of the magnetic objective 3 by providing at least one third hole 3e in the outer pole piece and at least one in the inner pole piece. adjoining fourth opening 3f. The third holes 3e are of the same shape and size as the first hole 3c and their number including the first hole 3c is n. The fourth holes 3f are analogously of the same shape and size as the second hole 3d and their number including the second hole 3d is m. and all openings 3c, 3e in the outer pole piece, as well as all openings 3d, 3f in the inner pole piece, are symmetrically spaced around the optical axis of the magnetic objective 3. In addition, in this case an even number of symmetrically spaced apertures other detectors of signals emitted from sample 4, such as a backscattered electron detector, a cathodoluminescence detector, an X-ray detector, and the like, which are disposed on the withdrawable device. During operation of the secondary electron detector 5, these additional detectors are in the retracted position and do not protrude from the inner pole of the first bipolar lens 3a. When used, they are extended into the optical axis region and the secondary electron detector 5 is turned off. This has the advantage that it allows to acquire signals from secondary or backscattered electrons or cathodoluminescence or X-rays at the same working distance.

The voltage at the extraction electrode 7 can also be used to further increase the resolution of the microscope, especially for lower kinetic energy values of the primary electrons. In this case, a high voltage of about 1 to 10 kV is used instead of the original 300 V extraction voltage. For example, if the extraction electrode 7 has a voltage of 3 kV and the primary electrons have a kinetic energy of 1 keV when entering the magnetic objective 3, the primary electrons are accelerated and pass through the gap between the 4 keV pole pieces. Thus, the optical imaging aberrations are lower and the resolution of the microscope is higher than if the primary electrons passed through the gap between the pole pieces with only 1 keV energy. As they pass through the outer, ie lower, pole piece, the primary electrons are retarded again and fall on sample 4 with their original energy of 1 keV. The optimum voltage at the extraction electrode 7 varies for the different kinetic energies of the primary electrons. However, with increasing energy, there is less and less difference between the resolution of the microscope without or with the use of high voltage on the extraction electrode 7. The effect of decelerating primary electrons upon impact on the sample can be further enhanced by applying a negative voltage of about (-1) to sample 4. - (- 10) kV. In this case, the electrode 7 may extend to the end of the outer (lower) pole piece because the primary electrons are retarded by the voltage on the sample 4.

In the case where a second two-pole lens 3b is used, it is not necessary to optimize the voltage at the extraction electrode 7 according to the energy of the primary electrons and only use low voltages to draw the secondary electrons into the magnetic objective 3.

-4GB 298912 B6

In the case where a combination of both the first and second bipolar lenses 3a and 3b is used, the voltage at the extraction electrode 7 can be optimized according to the excitation of the first bipolar lens 3a.

Advantageously, the entire secondary electron detection system, i.e. the extraction electrode 7, the optional retarding electrode 8 and the detector 5, are located below the scanning system 6. The scanning system 6 is configured to deflect a primary electron beam of energy of the order of keV. On the other hand, secondary electrons have energy in units or tens of eV, so that they would deviate much more from the optical axis when passing through the same rasters, which would cause considerable technical problems in their detection. It is technically advantageous to bias the secondary electrons towards the detector 51 below the scanning system 6, where they are concentrated relatively near the optical axis. However, the invention can also be used in the opposite arrangement of the detector 5 above the scanning system 6.

Fig. 2 shows a second embodiment. This time, part of the microscope on the objective, ie the electron source and the condensers, is omitted. The magnetic lens 3 again consists of a first bipolar lens 3a at a first working distance WD1 and a second bipolar lens 3b at a second working distance WD2, this time having connected magnetic circuits. In this case, the retarding electrode 8 is omitted. The primary electron beam is again focused on the sample 4 by the magnetic objective 3 and the scanning system 6 ensures its spacing on the surface of the sample 4.

The display properties are the same as described in the previous example.

The secondary electrons are again drawn into the magnetic objective 3 and accelerated by the extraction electrode 7. This time, however, they hit the target 10 where they emit tertiary electrons which are detected by the detector 5 whose signal is displayed on the terminal device 5a. The surface of the target 10 is a suitable material having a sufficiently high emissivity of the electrons. Heavy elements with an atomic number greater than 20 are suitable.

As can be seen from this example, the first and second holes 3c and 3d, respectively, for the secondary electron detector 5 can also be drilled in the inclined portion of the cone poles and may not be completely coaxial.

The examples shown so far have the theoretical disadvantage that the detector array 5 reaches the optical axis and thus causes optical defects of the primary electron beam. In practice, these optical defects can be kept to a negligible level because the detector array 5 around the optical axis is sufficient only to deflect secondary electrons by energy in eV units, so that the deflection of primary electrons with energy in thousands of eV will be small. Fig. 3 shows an example of another embodiment which solves this theoretical disadvantage. Between the extraction electrode 7 and the plane of the second aperture 3d is inserted a device 11 for generating a cross adjustable adjustable magnetic and electrostatic field perpendicular to the optical axis of the magnetic objective 3, the so-called Wien filter. The electrostatic field is perpendicular to the magnetic and sized so that for a given velocity of the primary electrons the electric force is as large, but in the opposite direction as the magnetic force. Thus, the primary electron beam is not bent by the Wien filter. On the other hand, the secondary electrons emitted from the sample 4 move in the opposite direction, and thus the electric and magnetic forces acting on them are in the same direction and deflect the secondary electrons into the detector 5. In this case on the optical axis. Again, between the Wien filter and the extraction electrode 7, the secondary electrons can be retarded by the retarding electrode 8, so that the Wien filter can produce a weaker field to deflect the secondary electrons toward the detector 5.

Fig. 4 shows another exemplary embodiment. This time, the retarding electrode 8, to which the negative voltage is applied or grounded, is located above the plane of the detector 5. The secondary electrons emitted from the sample 4 and accelerated by the extraction electrode 7 are impeded by the retarding electrode 8 in the plane of the second opening 3d and attracted by the detector field. FIG. 4 also illustrates a different method of controlling the field strength of the detector 5 around the optical axis than by simply varying the distance

The front of the detector 5 from the optical axis or its voltage by adding a deflector 12 to which a suitable positive control voltage is applied, e.g. about 200 V. The deflector 12 may be, for example, a grating or a sleeve-shaped electrode. In order to prevent the field of the retarding electrode 8 from undesirably interfering with the scanning system 6, it can be shielded by a grounded sleeve 13. In this example it can also be seen that the shape of the magnetic lens 3 is not limited to tapered. insert 5 secondary electrons under the detector.

These examples are illustrative only and can be combined in various ways, eg the shape of the magnetic lens 3 from one example can be used in another example and the like.

Industrial applicability

The microscope of the present invention can be used wherever high resolution and high field of view specimens need to be monitored, for example in the fields of material study, integrated circuit quality control, semiconductor industry, medicine, biology, nanotechnology, scientific applications or basic research.

Claims (16)

PATENT CLAIMS 25 1. A scanning electron microscope comprising a primary electron source, means for accelerating primary electrons to a desired kinetic energy, a magnetic objective having an optical axis, a scanning electron beam scanning system for a sample and a secondary electron detector, characterized in that the magnetic objective (3) ) consists of a first two-pole lens (3a) located closer to and to the measured sample (4) 30 (4) at a first working distance (WD1), and a second two-pole lens (3b) lying on a common optical axis with a second working distance (WD2) from the sample (4) and a secondary electron detector (5) is positioned between a first two-pole lens (3a) and a second two-pole lens (3b). Scanning electron microscope according to claim 1, characterized in that: 35 in the pole pieces of the first two-pole lens (3a) there are adjacent holes for receiving the secondary electron detector (5), in the outer pole piece the first hole (3c) and in the inner pole piece the second hole (3d). Scanning electron microscope according to claim 2, characterized in that: 40 further openings are formed in the pole pieces of the first two-pole lens (3a), such that at least one third opening (3e) is formed in the outer pole piece and at least one fourth opening (3f) adjoining the inner pole piece wherein the third apertures (3e) are of the same shape and size as the first aperture (3c), the number including the first aperture (3c) to the fourth apertures (3 f) being the same shape and size as the second aperture (3d) and their number including the second The openings (3d) are m, n and m are even numbers and all openings (3c, 3e) in the outer pole piece as well as all openings (3d, 3f) in the inner pole piece are symmetrically spaced around the optical axis of the objective. Scanning electron microscope according to any one of claims 1 to 3, characterized by: 50, characterized in that the scanning system (6) is located between the first two-pole lens (3a) and the second two-pole lens (3b) of the magnetic objective (3). -6GB 298912 B6 Scanning electron microscope according to claim 4, characterized in that the scanning system (6) lies above the secondary electron detector (5). Scanning electron microscope according to any one of claims 1 to 5, characterized by: 5, characterized in that an extraction electrode (7) connected to a source of positive, preferably variable, voltage is arranged in the magnetic objective (3), wherein the voltage at the pole pieces of the magnetic objective (3) is zero. Scanning electron microscope according to claim 6, characterized in that at least one retarding electrode (8) is placed above the extraction electrode (7). Scanning electron microscope according to claim 7, characterized in that the retarding electrode (8) is connected to a source of low negative voltage. 15 Dec Scanning electron microscope according to claim 7, characterized in that the retarding electrode (8) has an aperture shape and is grounded. Scanning electron microscope according to claim 8, characterized in that a grounded shielding sleeve (13) is arranged above the retarding electrode (8). Scanning electron microscope according to claim 6, characterized in that a target (10) having an opening for the passage of the primary electrons is located above the plane of the second opening (3d). 25 Scanning electron microscope according to any one of claims 1 to 9, characterized in that a device (11) for generating a crossed magnetic and electrostatic field, preferably adjustable, perpendicular, is arranged between the extraction electrode (7) and the plane of the second opening (3d). on the optical axis of the lens. 30 Scanning electron microscope according to any one of claims 1 to 12, characterized in that in front of the secondary electron detector (5) there is a separator (12) connected to a positive voltage source. Scanning electron microscope according to any one of claims 3 to 13, characterized by: 35, characterized in that a backscattered electron detector and / or a cathodoluminescence detector and / or an X-ray detector are inserted into at least one of the pairs of adjoining third orifices (3e) and the fourth orifice (3f). Scanning electron microscope according to any one of claims 1 to 14, characterized by: 40, characterized in that the first bipolar lens (3a) and the second bipolar lens (3b) have connected magnetic circuits. Scanning electron microscope according to any one of claims 1 to 15, characterized in that the sample (4) is connected to a negative voltage source.